Concrete block paving (CBP)

How does Concrete block paving (CBP) work, step by step?

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   Step 1: Prepare and prove the base

   Block paving reflects what is under it, so the base does the structural work and the blocks provide the surface. The formation, subbase and base - bound or unbound according to the designer's choice - are placed, compacted and surveyed to a tolerance tighter than for most other pavement types, because the thin laying course cannot regulate out irregularity. Drainage falls are set in the base rather than corrected in the laying course. Edge restraints, kerbs and channels are built before laying, because a block pavement without restraint at its edges will spread and fail there first.

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   Step 2: Complete the services and record them

   Reefer ducts, lighting and crane supplies, drainage connections, chambers and draw pits are installed and surveyed before laying begins. Block paving is more forgiving here than concrete because access can be regained later, but the as-built record still matters - the operator lifting blocks in five years wants to know exactly where to lift. Chamber and gully surrounds are detailed carefully, because the junction between a rigid frame and a flexible block surface is where most block yards first show distress.

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   Step 3: Lay and screed the laying course

   The laying course of sand or grit is spread and screeded to a consistent thickness, and it is protected from rain and from being walked or trafficked before the blocks go on. Consistency matters more than absolute thickness, because the blocks will settle by a predictable amount when they are compacted and any variation in the laying course becomes a variation in the finished surface. Screeded material is laid only as far ahead of the block layers as can be covered the same day.

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   Step 4: Lay the blocks to the specified pattern

   Blocks are laid to the pattern the designer has specified, laid tight with narrow joints, working forward off the laid surface so that the screeded laying course is never walked on. Herringbone patterns are usual under heavy trafficking because they resist creep under braking and turning far better than stack or stretcher patterns. Laying lines are checked frequently against stringlines or a laser, because a block pavement that starts to run out of line will not correct itself. Blocks are drawn from several packs at once so that any colour or shade variation is distributed rather than banded.

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   Step 5: Cut the edges and complete the details

   Perimeter blocks, and those around gullies, chambers, bollards and crane rails, are cut to fit with a block splitter or saw. Cut pieces below the size the specification allows are not used, because small slivers work loose under traffic almost immediately. Detailing around fixed items is where a block yard is judged, and it is worth the time it takes - a ragged edge around a chamber becomes a maintenance liability within a season.

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   Step 6: Fill the joints and compact the pavement

   Jointing sand is swept into the joints and the pavement is vibrated with a plate compactor fitted with a protective mat, which forces sand down into the joints and beds the blocks into the laying course. Sand is swept in and the compaction repeated until the joints are full to the top. This is not a finishing operation - it is what creates the interlock that makes the pavement work, and an under-filled joint is a structural defect rather than a cosmetic one.

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   Step 7: Check level, regularity and joint width

   The finished surface is surveyed for level and regularity against the specification, and joint widths are checked against the tolerance. Areas that are out of tolerance are lifted and relaid, which is quick and cheap compared with the equivalent operation in any other pavement type. Falls are re-checked to confirm the yard drains as designed, with particular attention around gullies and channels where ponding will otherwise appear.

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   Step 8: Top up the joints and hand over the maintenance regime

   Joints are topped up again shortly after the area is first trafficked, because early traffic settles the sand further. The operator is handed a maintenance regime that makes the joint sand obligation explicit: periodic inspection, topping up, care with mechanical sweeping and washing, and prompt attention to any area where blocks begin to rock. Spare blocks from the same production are left with the operator so that later repairs and relaying match the original.